Effective Nucleation Host Alloys (ENHAs): A New Materials Solution for Stable and Effective Li Metal Anodes

Abstract

There is a global demand for high energy storage systems for portable consumer devices of cell phones, laptops, including electric vehicles. Lithium ion batteries (LIBs) are the most promising options due to the high volumetric, gravimetric energy, and power densities. Current LIBs using LiCoO2, LiNiO2, LiMnO4, and LiFePO4 cathodes coupled with a graphite anode exhibit a power density of 100-260 Whkg-1 [1]. Recent studies on high capacity Li – S batteries indicate their promise for replacing low capacity carbon anodes with Li metal and composites. However, Li metal plating on Cu/Li substrates shows undesirable performance due to Li plating instabilities leading to formation of high surface area Li, loss of Li due to uneven plating/deplating, increase in internal resistance and formation of deleterious Li dendrites[2-4]. The fundamental plating phenomena, result in low columbic efficiencies, increase in plating/deplating potentials and consequent hysteresis losses causing rapid fade in specific capacity/energy density and thermal run away either due to increased internal resistance or internal short circuiting of the cell due to formation and growth of unwanted dendrites. To address the above issues, novel structurally isomorphous alloys (SIAs) were developed to replace the copper/carbon systems with their ability to modify the Li plating morphology by altering the nucleation and growth mechanisms. These alloys comprise effective host alloys (EHAs) with similar crystallographic registry as Li serving as the Li source. Different effective nucleating agents (ENAs) are alloyed with EHAs to form novel effective nucleating host alloys (ENHAs) transforming the Li plating phenomena towards the uniform homogenous nucleation and growth regimes. High energy mechanical milling (HEMM) was used to synthesize these new materials and the evolution of the phases was studied using X-ray diffraction. The EHAs as the anode system with a hexagonal close packed crystal (HCP) structure shows inferior overall performance albeit eliminating dendrite formation. However, ENHAs formed by alloying EHAs with ENAs with the body-centered cubic structure, display the ability of ENHAs to act as an effective Li source for plating/de-plating. The ENHAs when studied for Li plating/de-plating in a CR2025 coin cell using Li as the counter/reference electrode and 1M LiPF6 in EC:DEC:FEC as the electrolyte at different areal current/charge densities, improved reversible Li plating/de-plating with a high coulombic efficiency of ~99.6% at the end of 200 cycles is observed (Fig 1a). The plating/de-plating potential decreases ~20-40 mV as the electrode stabilizes after 40 cycles (Fig 1b). Post cycling SEM analysis (Fig 1c) reveal absence of dendritic morphologies due to efficient nucleation and uniform growth during Li plating. Different alloy compositions of ENHAs are studied for lithium plating and their phase evolution, electrochemical performance and post cycling SEM analysis results will be presented and discussed.